operator delete should check for null pointer before deallocating

It is a pervasive belief that using "delete" with a null pointer is safe, so our custom delete operators should also handle this case correctly.

This may fix regressions introduced by #934

direct/src/dcparser/dcPacker.I

@@ -1124,7 +1124,9 @@ operator new(size_t size) {
  */
 INLINE void DCPacker::StackElement::
 operator delete(void *ptr) {
-  StackElement *obj = (StackElement *)ptr;
-  obj->_next = _deleted_chain;
-  _deleted_chain = obj;
+  if (ptr != nullptr) {
+    StackElement *obj = (StackElement *)ptr;
+    obj->_next = _deleted_chain;
+    _deleted_chain = obj;
+  }
 }

dtool/src/dtoolbase/deletedChain.h

@@ -85,7 +85,9 @@ public:
     return ptr;                                              \
   }                                                          \
   inline void operator delete(void *ptr) {                   \
-    StaticDeletedChain< Type >::deallocate((Type *)ptr, get_type_handle(Type)); \
+    if (ptr != nullptr) {                                    \
+      StaticDeletedChain< Type >::deallocate((Type *)ptr, get_type_handle(Type)); \
+    }                                                        \
   }                                                          \
   inline void operator delete(void *, void *) {              \
   }                                                          \
@@ -104,7 +106,9 @@ public:
     return ptr;                                              \
   }                                                          \
   inline void operator delete(void *ptr) {                   \
-    _deleted_chain.deallocate((Type *)ptr, get_type_handle(Type)); \
+    if (ptr != nullptr) {                                    \
+      _deleted_chain.deallocate((Type *)ptr, get_type_handle(Type)); \
+    }                                                        \
   }                                                          \
   inline void operator delete(void *, void *) {              \
   }                                                          \

dtool/src/dtoolbase/memoryBase.h

@@ -32,7 +32,9 @@
     return ptr;                                              \
   }                                                          \
   inline void operator delete(void *ptr) {                   \
-    PANDA_FREE_SINGLE(ptr);                                  \
+    if (ptr != nullptr) {                                    \
+      PANDA_FREE_SINGLE(ptr);                                \
+    }                                                        \
   }                                                          \
   inline void operator delete(void *, void *) {              \
   }                                                          \
@@ -44,7 +46,9 @@
     return ptr;                                              \
   }                                                          \
   inline void operator delete[](void *ptr) {                 \
-    PANDA_FREE_ARRAY(ptr);                                   \
+    if (ptr != nullptr) {                                    \
+      PANDA_FREE_ARRAY(ptr);                                 \
+    }                                                        \
   }                                                          \
   inline void operator delete[](void *, void *) {            \
   }